There is a need in industry for reactive metal granules and, in particular, for granules of Mg or Mg alloy for the treatment of steel, aluminum or other metals and for other purposes such as thixotropic injection moulding. These applications require granules of at least 1 mm in size and the granules should be substantially free of surface oxides. For some uses, granules coated with a layer protecting them from oxidation may be advantageously used and various salts, for example, have provided this advantage.
There are few commercial processes which directly produce reactive metal granules. For many applications, such granules are produced by cutting or shearing material from larger pieces of metal.
U.S. Pat. No. 4,457,775 issued on Jul. 3, 1984 Legge et al. discloses a method for producing Mg granules by mixing Mg into a salt bath of specific composition with agitation, then partially separating the product from the bath to obtain a salt/granule mixture. Because of the production method, the composition is somewhat variable.
Metal granules or shot from less reactive metals (iron, steel, copper, etc.) have been produced by injection from a nozzle into liquid baths or into counter-current gas streams. The former process is a difficult operation for a reactive metal and the latter process requires a spray tower of substantial height, and is limited in practice to granules of small diameter because of cooling considerations.
Furthermore, in order to be adapted to reactive metals, substantial quantities of inert gas would be required.
PCT publication WO-A-86 06013 (and equivalent U.S. Pat. No. 4,915,729) disclose a process in which a molten metal is contacted with a bed of moving beads. The molten metal breaks up into fine particles which are rapidly cooled in contact with the beads. However, the mechanical agitation produces particles of metal in the form of angular flakes rather than spherical granules. The method is not well suited to the formation of particles of reactive metals, since the large surface area tends to encourage oxide formation and reaction.
There is accordingly a need for an improved method of producing granules of reactive metals.